This invention relates to a photomultiplier tube, and more particularly to a photomultiplier tube having a venetian-blind type dynode.
A photomultiplier tube has been conventionally utilized to detect light having weak intensity into an amplified electrical signal. The photomultiplier tube basically includes a photocathode for converting light incident thereto into photoelectrons having information corresponding to the intensity of the light, a dynode array comprising plural dynode elements (vanes) for emitting secondary electrons at a predetermined multiplication rate upon incidence of an electron, an anode array for collecting the multiplied secondary electrons emitted from the dynode array and outputting an electrical signal to thereby convert the light having weak intensity into the amplified electrical signal corresponding thereto, and an envelope for accommodating the photocathode, the dynode array and the anode array.
In order to improve sensitivity and resolution of the photomultiplier tube, there have been hitherto proposed various structures for the dynode array such as a mesh type in which plural mesh-shaped dynodes are arranged in a longitudinal direction of the envelope, a venetian-blind type in which plural plate-shaped dynodes are arranged in the longitudinal direction of the envelope and so on.
The photomultiplier tube having the mesh type of dynode array is described in U.S. Pat. No. 4,937,506. In this type of photomultiplier tube, photoelectrons emitted from the photocathode are first bombarded against wires of a first mesh-shaped dynode to emit secondary electrons therefrom, and then the secondary electrons are successively bombarded against the successive mesh-shaped dynodes to multiply the secondary electrons. In this type of dynode array, wires constituting the mesh-shaped dynodes (that is, effective areas of the dynodes for receiving photoelectrons and emitting secondary electrons upon incidence of photoelectrons) are extremely small and narrow, and thus it is difficult to control a photoelectron stream emitted from the photocathode to concentrically impinge on the respective wires of the dynodes to improve multiplication efficiency. This photo-multiplier tube is equipped with a mesh-shaped electrode disposed in contact with the photocathode and kept fixedly at the same potential as the photocathode. This electrode is used to prevent spread of the photoelectrons emitted from the surface of the photocathode, but has no function of controlling the photoelectron stream to concentrically impinge to the wires (effective secondary electron emission areas of the dynodes).
The photomultiplier tube having the venetian-blind type of dynode array is shown in FIG. 1. The venetian-blind type of dynode array includes dynode elements each having a larger effective area for receiving photoelectrons and emitting secondary electrons upon incidence of the photoelectrons than the mesh type of dynode array because each dynode element of the venetian-blind type is of a plate type, so that the collection and emission efficiency of the electrons in the venetian-blind type of dynode array is better than that of the mesh type of dynode array.
As shown in FIG. 1 the photomultiplier tube of this type basically includes an elongated glass envelope 1 having a flat plate type light-incident surface 2 for passing an incident light therethrough to an inner side thereof, a photocathode 3 provided at the inner wall of the light-incident surface 2 for converting the incident light into photoelectrons, plural mesh electrodes 4.sub.1 to 4.sub.n and plural dynode elements (vanes) 7 having a venetian-blind structure in that plural dynode rows 5.sub.1 to 5.sub.n each comprising plural dynode elements arranged horizontally at a constant interval are vertically arranged at a constant interval as shown in FIG. 1, the mesh electrodes and the dynode rows being vertically and alternately arranged along a longitudinal direction of the glass envelope 1 to form a multi-stage arrangement, and an anode array comprising plural anodes 6 arranged horizontally in such a manner as to confront the dynode elements of the last dynode row (the bottom dynode row) at the last stage and are connected to terminals to output an external circuit (not shown).
Each dynode element comprises a plate type of electrode element having a shorter width (for example, a strip form), which is elongated in a direction vertical to the surface of the drawing. Each of the dynode elements is inclined to the longitudinal direction of the envelope 1 (in the vertical direction) as shown in FIG. 1. The inclining direction of the dynode elements is alternately changed at the respective stages. For example, all dynode elements of the dynode rows at the odd stages are inclined to the longitudinal direction of the envelope 1 by approximately 45 degrees in a clockwise direction, while all dynode elements of the other dynode rows at the even stages are inclined to the longitudinal direction of the envelope 1 by approximately 45 degrees in a counterclockwise direction (in the direction opposite to that of the odd stages).
In the photomultiplier tube thus constructed, the photocathode 3 is supplied with a voltage of 0 (volt), and a first pair of the mesh electrode (4.sub.1) and the dynode row (5.sub.1) at the first (uppermost) stage is supplied with approximately 300 (volts). A second pair of the mesh electrode (4.sub.2) and the dynode row (5.sub.2) at a second stage and the successive pairs of the mesh electrodes (4.sub.3 to 4.sub.n) and the dynode rows (5.sub.3 to 5.sub.n) at the successive stages are supplied with an incremental voltage which is successively increased by every 100 volts with respect to the voltage to be supplied to the first pair. The anode array is supplied with a highest voltage (for example, 1300 volts).
Upon incidence of light to a position 3f on the photocathode 3 in the venetian-blind type of photomultiplier tube, photoelectrons are emitted from the photocathode 3 and then are multiplied as secondary electrons by the first and successive dynode rows. Idealy, the multiplied secondary electrons should be detected by an anode 6f disposed at a position corresponding to the light-incident position 3f. However, in this type of photomultiplier tube, an electron stream of photoelectrons emitted from one point of the photocathode 3 spreads due to both of variation in energy of photoelectrons emitted from the surface of the photocathode 3 and a cosine-distributed emission angle thereof. The variation in energy of the photoelectrons is caused by difference in energy loss of the photoelectrons through a travel within the photocathode. That is, the photoelectrons are emitted in various positions different in depth of the photocathode (a photoelectron emitting layer), and thus lose different amounts of energy through collision with atoms from generation thereof till emission thereof from the surface of the photocathode. On the other hand, the cosine-distributed emission angle is caused by difference in emission angle of respective photoelectrons with respect to the surface of the photocathode. This spread in the electron stream disturbs all emitted secondary electrons from being detected by an anode corresponding to the light-incident point of the photocathode. In other words, some secondary electrons are not detected by the anode, but by other anodes disposed near to the anode as shown in FIG. 1, so that cross-talk is liable to occur.
A discriminating characteristic of this photomultiplier tube was estimated in the following manner: the light-incident surface 2 and the photocathode 3 are scanned with a spot light 10 of sufficiently-small diameter from a left side to a right side in FIG. 5, and an output signal is detected by only a specific anode 6f disposed at the center portion of the anode array.
FIG. 2 is a graph showing the discriminating characteristic obtained by the above manner, in which abscissa and ordinate represent a relationship between a position on the photocathode 3 to be scanned with a small spot of light and a relative value of an output signal from the anode 6f. In FIG. 2, a hatched portion of the graph represents a cross-talk occurring in the output signal, and particularly the hatched portion profiled by a dotted line B represents a cross-talk occurring in the conventional photomultiplier tube.
Further, in the conventional photomultiplier tube thus constructed, those secondary electrons which are upwardly emitted from the dynodes 5.sub.1 at the first stage, particularly from upper portions 7a of the dynode elements 7 of the first dynode row 5.sub.1, are upwardly passed through the first mesh electrode 4.sub.1 and then returned to the dynode elements of the first dynode row 5.sub.1. That is, some secondary electrons emitted at the upper portions 7a are not immediately and directly directed to the dynode elements at the second stage. On the other hand, other secondary electrons which are emitted from the lower portions 7b are immediately and directly directed to the dynode elements at the second stage with no disturbance. That is, the secondary electrons emitted from the upper portions 7a of the first stage are bombarded against the secondary dynode row later than those emitted from the lower portions 7b of the first stage, there occurs a difference in flight time between these two types of secondary electrons even though they are emitted from the same dynode element at the first dynode row 5.sub.1. This difference in flight time of the secondary electrons emitted from the same dynode element causes a time scattering (time dispersion) of an output signal. The difference in flight time of the secondary electrons emitted from the first dynode row is approximately 3 nanoseconds, and causes the timing resolution to be degraded.